There has been proposed an image reading device mounted on a copying device, a scanning device or a multi-function device having copy functions and scan functions. The image reading device has an automatic document feeder (ADF) which nips and conveys a document to a discharge tray through the conveying path from the sheet teed tray. Further, there has been proposed an automatic document feeder which carries out conveying for reading the document at a midpoint of conveyance by reversing leading and trailing ends of a document by switching the document back at the midpoint of conveyance in order to read image sprinted on both first and second sides of the document (refer to JP-A-10-87108, for example).
FIG. 15 shows a conveying route of a related image reading device having an ADF capable of reading both sides of a document. As shown in FIG. 15, a document P, which is placed on a sheet feed tray 100 with the first side (page 1) facing upward, is conveyed to a conveying path 102 by a sheet feed roller 101. In the conveying path 102, the document P is conveyed to the conveying rollers 103 mounted appropriately, and the first side of the document P is read by an image reading unit such as a reduced-optical charge coupled device (CCD) or a contact image sensor (CIS) while passing through a reading position X. When the trailing end of the document P after the first side has been read is detected by a sensor, a sheet discharge roller 104 is halted when the document is nipped in the vicinity of the trailing end of the document P.
As shown in FIG. 16, the sheet discharge roller 104 is reversed, and the document P is conveyed to a switchback path 105. The document P advances again from the switchback path 105 to a position upstream of the reading position X on the conveying path 102. Accordingly, the leading end of the document P and the trailing end are reversed with each other. Then, the document P is conveyed by the conveying rollers 103, and the second side of the document P is read by the image reading unit while passing through the reading position X. When the trailing end of the document P after the second side has been read is detected by a sensor, the sheet discharge roller 104 is again halted when the document is nipped in the vicinity of the trailing end. Thereafter, the document P is conveyed reversely through the switchback path 105. The document P which advances again from the switchback path 105 into the conveying path 102 is in a state that the trailing end and the leading end of the document are again reversed with each other, that is, the first side is faced to the reading position X. Then, the document P is conveyed through the conveying path 102 and discharged to the sheet discharge tray 106, with the first side facing downward. Thereby, the first side and the second side of the document P are read and the document P is discharged to the sheet discharge tray 106 sequentially in the order placed on the sheet feed tray 100.
In the conveying process of document P by the ADF, the document P passing through the reading position X is conveyed respectively being nipped by the conveying rollers 103 mounted at the respective positions upstream and downstream from the reading position X. In general, the downstream conveying roller 103 is set to be greater in peripheral speed (speed on the roller face) than the upstream conveying roller 103. Thereby, the document P is conveyed in a way of being pulled by the downstream conveying roller 103 at the reading position X and therefore no curling is developed in the documents P.
When the trailing end of a document P in a sheet feed direction passes through the upstream conveying roller 103 at the reading position X, tension applied to the document P by the respective conveying rollers 103 upstream and downstream at the reading position is released. Thus, it is considered that behavior of the document P become unstable. In general, a difference in the peripheral speed between the respective conveying rollers 103 upstream and downstream from the reading position is set within a range which will not cause the above-described unstable behavior of the document P. However, when a sheet discharge roller 104 is arranged further downstream from the downstream conveying roller 103, the peripheral speed of the sheet discharge roller 104 may affect the behavior of the document P. Further, when a conveying path 102 is configured differently or a switchback path is provided separately, the behavior of the document P may be changed by the configuration of the conveying path downstream from the reading position. Therefore, a combination of factors influencing the behavior of the document P results in unstable behavior of the document P at the reading position. There is a fear that image reading of the document P may be affected by an image reading unit. However, factors influencing behavior of the document P substantially remain unknown. Thus, it is difficult to feed the document P at the reading position taking every factor into account.